Question

我目前有一个相当简单的数据库，由脚本定期更新。我想通过比较2列的相等性来过滤结果。 SQL查询看起来像：

SELECT devices.site_name, devices.device_name, devices.created_at
FROM devices, locations 
WHERE devices.created_at = locations.most_recent_updated_time AND devices.site_name = locations.site_name;

这是我的datamapper模型

class Device
  include DataMapper::Resource

  property :id, Serial
  property :device_name, Text, :required => true
  property :site_name, Text, :required => true
  property :created_at, DateTime

  belongs_to :location 

end

class Location
  include DataMapper::Resource

  property :site_name, Text, :required => true, :key => true
  property :most_recent_updated_time, DateTime, :required => true

  has n, :devices
end

每次脚本运行时，它都会附加新的Device条目并更新Location表中的`：most_recent_updated_time字段。以下是表格更新方式的示例。

Devices
╔════╦═════════════╦═══════════╦═══════════════════════════╗
║ ID ║ Device_Name ║ Site_Name ║        Created_at         ║
╠════╬═════════════╬═══════════╬═══════════════════════════╣
║  1 ║ Device1     ║ Dallas    ║ 2013-07-23T16:59:12-04:00 ║
║  2 ║ Device2     ║ Dallas    ║ 2013-07-23T16:59:12-04:00 ║
║  3 ║ Device3     ║ Chicago   ║ 2013-07-23T16:59:12-04:00 ║
╚════╩═════════════╩═══════════╩═══════════════════════════╝
Location
╔═══════════╦═══════════════════════════╗
║ Site_Name ║ Most_Recent_Updated_Time  ║
╠═══════════╬═══════════════════════════╣
║ Dallas    ║ 2013-07-23T16:59:12-04:00 ║
║ Chicago   ║ 2013-07-23T16:59:12-04:00 ║
║ Boston    ║ 2013-07-23T16:59:12-04:00 ║
╚═══════════╩═══════════════════════════╝

脚本运行后

Devices
╔════╦═════════════╦═══════════╦═══════════════════════════╗
║ ID ║ Device_Name ║ Site_Name ║        Created_at         ║
╠════╬═════════════╬═══════════╬═══════════════════════════╣
║  1 ║ Device1     ║ Dallas    ║ 2013-07-23T16:59:12-04:00 ║
║  2 ║ Device2     ║ Dallas    ║ 2013-07-23T16:59:12-04:00 ║
║  3 ║ Device3     ║ Chicago   ║ 2013-07-23T16:59:12-04:00 ║
║  4 ║ Device4     ║ Dallas    ║ 2013-07-24T18:00:00-04:00 ║
║  5 ║ Device5     ║ Chicago   ║ 2013-07-24T18:00:00-04:00 ║
╚════╩═════════════╩═══════════╩═══════════════════════════╝
Location
╔═══════════╦═══════════════════════════╗
║ Site_Name ║ Most_Recent_Updated_Time  ║
╠═══════════╬═══════════════════════════╣
║ Dallas    ║ 2013-07-24T18:00:00-04:00 ║
║ Chicago   ║ 2013-07-24T18:00:00-04:00 ║
║ Boston    ║ 2013-07-24T18:00:00-04:00 ║
╚═══════════╩═══════════════════════════╝

因此，如果我在脚本运行后运行查询，我希望它返回Device4和Device5。

Answer 1

如果我理解您的问题，请尝试将此方法添加到Location：

def most_recent_devices
  devices.all(:created_at => most_recent_updated_time)
end

然后你应该能够收集每个Location的所有最新设备，如下所示：

Location.all.map(&:most_recent_devices).flatten

Ruby / Datamapper查询比较来自不同表的列

1 个答案: